This invention relates generally to transducers, and more particularly to a method of calibrating acoustic emission transducers.
The use of electroacoustic sensor elements as flaw detectors for metal vessels is widespread. In its simplest form, a piezoelectric element is acoustically coupled to the medium which is to be monitored, and the electrical signal derived therefrom indicates the condition of the medium under study. Acoustic emission transducers have been proposed for use as passive listening devices to detect the noise being emitted by growing flaws as, for example, in monitoring the metal wall of a nuclear reactor pressure vessel. Such electroacoustic transducers are affixed to the exterior pressure vessel wall, and remain in place for monitoring of the vessel wall condition during operation. However, in order to be useful for nuclear reactor service, the sensitivity of the acoustic emission transducer at the various monitoring frequencies must be determined when acoustically loaded by a medium representative of a nuclear reactor pressure vessel. Without such calibration, meaningful data would not be supplied by the acoustic emission transducer.
One method used to calibrate acoustic emission transducers consists of coupling the face of the acoustic emission transducer directly against the face of a transmitting transducer having a flat transmitting response, and electrically driving the transmitting transducer in the fashion of a loudspeaker. The receiving response of the acoustic emission transducer is then measured. However, when applied to transducers intended for nuclear reactor use, this technique gives results that are in disagreement with those obtained during field tests. The disagreement in results is caused by the transducers not being loaded by a representative acoustic medium during calibration.
Another method utilized to calibrate acoustic emission transducers is to mount the transducers on a long, thin bar, and to excite the bar with spark-generated simulated acoustic emission pulses. While the impulsive nature of the spark-generated sound is closer to that generated in the nuclear reactor than with the first method, this second method also is prone to the same problem as is the first method; namely, the acoustic loading is unrepresentative of the actual loading which will be experienced during use.